Organic EL devices are a device employing a film material that emits light when excited by applying a current thereto. Since organic EL devices emit highly bright light at low voltage, these devices have a wide variety of potential applications including a cellular phone display, a personal digital assistant (PDA), a computer display, an information display of a car, a TV monitor, illuminating devices, and the like, and have such advantages as reducing the thickness or weight of these items or saving energy to operate them. Therefore, organic EL devices are expected to play a leading part in the future electronic device market. However, in order to practically apply these items employing an organic EL device instead of conventional displays, the brightness, color tone, endurance in various environmental conditions, or production cost or amount of the organic EL devices, have yet to be improved.
One characteristic of an organic EL device is that light of various colors can be produced by mixing light of plural colors.
In particular, there is a high demand for organic EL devices that emit white light, which may contribute to reduce energy consumption of illuminating devices, and are usable as an in-car display or a backlight. Further, by splitting its white light into blue, green and red pixels using a color filter, the organic EL devices that emit white light can also be used in full-color display devices.
On the other hand, since there are few materials that white light per se, combining plural materials that emit light of different colors, i.e., having different levels of exciton energy, is typically conducted to produce white light. In this case, as a result of using plural luminescent materials having different levels of exciton energy, transfer of energy from a material having a higher exciton energy to a material having a lower exciton energy is known to occur, and this may cause extinction or change of color of the light.
In order to address this problem, a method of isolating the luminescent materials from each other is proposed to prevent the energy transfer among them so that each luminescent material can emit the predetermined light.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2000-340361 proposes a method of depositing luminescent materials that emit light of different colors on a substrate to a thickness of 4 nm or less in the form of islands or stripes. In this method, however, there are technical difficulties in uniformly distributing the luminescent materials that emit light of different colors in the form of islands within a plane. Alternatively, if the luminescent layer is formed simply in the form of stripes, there is a fear that the transfer of energy among the adjacent layers may occur. Moreover, if the luminescent layer is extremely thin, durability of the luminescent layer may deteriorate due to the concentration of charges or electric fields thereto.
JP-A No. 2003-187977 proposes a method of producing white light by mixing light of different colors, by forming a three-layer structure in which a luminescent layer that emits light of a short wavelength is sandwiched by two luminescent layers that emit light of a long wavelength.
However, in such a structure as above in which luminescent layers that emit light of different colors (having different energy levels) are simply layered, further improvements are still to be made to address the problem of color change, which is caused by the change of location at which electrons and holes are recombined to emit light, due to the change of voltage applied to the device or the like.
In view of the above, the present invention provides an organic EL device that emits light of mixed colors with suppressed change in chromaticity caused by displacement of the location for light emission. In particular, the present invention provides an organic EL device that can be produced by a simple process and emits white light by combining three colors of luminescent devices having different luminescence peaks, such as blue, green and red, while suppressing the displacement of the location for light emission due to changes in driving voltage.